For facilitating interaction between users and electronic devices, electronic devices are generally provided with user input interfaces. So far, input keys has been one of the most popular user input interfaces. Even though remote controllers and touch sensing panels have been commonly used nowadays for input control, mechanical push buttons are still essential to general electronic devices for power switching, volume adjustment, brightness control, channel tuning, signal source selection, and so on. Since functions of a modern electronic device to be controlled via push buttons are diverse, some of the push buttons might be redundant for some applications. In addition, the disposition of conventional mechanical push buttons on a housing of a modern electronic device which has a thinned frame is difficult. Moreover, different models of products may need different numbers and allocations of push buttons, manufacturers would suffer from large stocks and high cost.